1. Field of the Invention
The present invention relates to a cell voltage detection device for detecting voltages of fuel cells of a fuel battery stack formed with a plurality of fuel cells serially connected to each other, and to the cell system using a cell voltage detection device.
2. Description of the Prior Art
Recently, fuel cells have been considered as an energy source with reduced environmental load. For example, the use of a polymer electrolyte fuel cell (PEFC) has been studied as an energy source of a cogeneration system using its heat and electric power or as a power source of an electric vehicle.
The fuel cell is a device that obtains electromotive force by electrochemically reacting the fuel gas containing hydrogen as a major component and the oxidant gas. The electromotive force of each fuel cell is at most only about 0.7 V. Thus, it is generally used a single fuel cell stack formed by laminating a dozen to hundreds of cells. The voltages of the stacked fuel cells vary depending on the distributions of fuel gas density, humidity and temperature within the stack, so that the voltage degradation tendency is different for each cell. As the reduction of each cell voltage is likely to have an effect on the life and safety of the stack, it is necessary to monitor the state of each cell voltage. As an example of the cell voltage detection device for detecting each of the voltages of the serially connected cells, a method for detecting the cell voltage of a secondary cell using an A/D converter element is disclosed, for example, in Japanese Patent Laid-open No. 2003-243044.
In order to operate the A/D converter element, it is necessary to supply a predetermined operating power source and a reference voltage for A/D conversion, to the A/D converter element. Here, the operating power source and the reference voltage are supplied using a part of the power of the cell whose voltage is to be measured. In this way, a device for externally supplying power can be eliminated. The secondary cell such as an Li-Ion cell can keep the cell voltage within a desired variation range by adjustment of the charge/discharge amount, if the self-discharge is ignored.